Use SPIRV-Tools commit ID as validation cache version
diff --git a/scripts/external_revision_generator.py b/scripts/external_revision_generator.py
new file mode 100644
index 0000000..acfb42e
--- /dev/null
+++ b/scripts/external_revision_generator.py
@@ -0,0 +1,82 @@
+#!/usr/bin/env python3
+#
+# Copyright (c) 2015-2016 The Khronos Group Inc.
+# Copyright (c) 2015-2016 Valve Corporation
+# Copyright (c) 2015-2016 LunarG, Inc.
+# Copyright (c) 2015-2016 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# Author: Cort Stratton <cort@google.com>
+
+import os
+import subprocess
+import sys
+
+
+
+if __name__ == '__main__':
+    if (len(sys.argv) != 4):
+        print("Usage: %s <SPIRV_TOOLS_SOURCE_DIR> <SPIRV_TOOLS_REVISION_FILE> <OUTPUT_HEADER_FILE>" % sys.argv[0])
+        sys.exit(os.EX_USAGE)
+    
+    spirv_tools_source_dir = sys.argv[1]
+    spirv_tools_revision_file = sys.argv[2]
+    output_header_file = sys.argv[3]
+    
+    # Load SPIRV-Tools revision
+    with open(spirv_tools_revision_file, "r") as rev_file:
+      revision = rev_file.read().replace('\n', '')
+      
+    # The revision listed in the file may be symbolic; use "git rev-parse" to
+    # convert it to a commit hash
+    spirv_tools_commit_id = subprocess.check_output(["git", "rev-parse", revision], cwd=spirv_tools_source_dir).decode('utf-8').strip()
+    
+    # Write commit ID to output header file
+    with open(output_header_file, "w") as header_file:
+         # File Comment
+        file_comment = '// *** THIS FILE IS GENERATED - DO NOT EDIT ***\n'
+        file_comment += '// See external_revision_generator.py for modifications\n'
+        header_file.write(file_comment)
+        # Copyright Notice
+        copyright = ''
+        copyright += '\n'
+        copyright += '/***************************************************************************\n'
+        copyright += ' *\n'
+        copyright += ' * Copyright (c) 2015-2017 The Khronos Group Inc.\n'
+        copyright += ' * Copyright (c) 2015-2017 Valve Corporation\n'
+        copyright += ' * Copyright (c) 2015-2017 LunarG, Inc.\n'
+        copyright += ' * Copyright (c) 2015-2017 Google Inc.\n'
+        copyright += ' *\n'
+        copyright += ' * Licensed under the Apache License, Version 2.0 (the "License");\n'
+        copyright += ' * you may not use this file except in compliance with the License.\n'
+        copyright += ' * You may obtain a copy of the License at\n'
+        copyright += ' *\n'
+        copyright += ' *     http://www.apache.org/licenses/LICENSE-2.0\n'
+        copyright += ' *\n'
+        copyright += ' * Unless required by applicable law or agreed to in writing, software\n'
+        copyright += ' * distributed under the License is distributed on an "AS IS" BASIS,\n'
+        copyright += ' * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n'
+        copyright += ' * See the License for the specific language governing permissions and\n'
+        copyright += ' * limitations under the License.\n'
+        copyright += ' *\n'
+        copyright += ' * Author: Chris Forbes <chrisforbes@google.com>\n'
+        copyright += ' * Author: Cort Stratton <cort@google.com>\n'
+        copyright += ' *\n'
+        copyright += ' ****************************************************************************/\n'
+        header_file.write(copyright)
+        # Contents
+        contents = '#pragma once\n\n'
+        contents += '#define SPIRV_TOOLS_COMMIT_ID "' + spirv_tools_commit_id + '"\n'
+        header_file.write(contents)
+    
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